In general, a fuel cell is an electricity generation apparatus for converting chemical energy into electric energy by using the oxidization and deoxidization of hydrogen and oxygen. Hydrogen is oxidized at an anode to be separated into hydrogen ions and electrons. While the hydrogenions are transferred to a cathode through an electrolyte, the electrons are transferred to the cathode through a circuit. The deoxidization occurs at the cathode. That is, the hydrogen ions, electrons and oxygen react with each other to generate water.
An amount of water contained in a polymer electrolyte membrane of the fuel cell, particularly, a polymer electrolyte membrane fuel cell (PEMFC) is one of the important factors deciding the performance of the fuel cell. It is because water serves as a medium transferring hydrogen ions to the cathode.
In addition to water, a temperature must be carefully controlled. When a reaction occurs in the fuel cell, a lot of heat is generated due to an activation loss, deoxidization at the cathode, and Joule heating. While activating a catalyst, such heat accelerates moisture reduction of the electrolyte membrane, to reduce ion conductivity. If the electrolyte membrane is exposed to a high temperature for a long time, durability of the electrolyte membrane decreases. Accordingly, besides the water management and the durability improvement, the temperature control is also an important factor deciding the performance of the fuel cell.
Normally, a cooling method using a coolant is employed to control the temperature of the fuel cell. A conventional fuel cell uses a separator made of a graphite material or a metal material. While a coolant channel is formed on the graphite separator by milling processing, it is formed on the metal separator by press processing. Passages of a reaction face and a cooling face can be individually manufactured on the graphite separator.
However, a reaction gas channel is stamp-processed on the metal separator, and its shape is reflected onto the opposite face as it is. It is thus difficult to individually form a passage of a coolant. Furthermore, the coolant channel is narrow and crooked, so that the coolant cannot smoothly flow in or out.
In order to solve the foregoing problems, a structure of introducing a coolant by forming a buffering region between a manifold and a channel is used to smoothly introduce the coolant into the metal separator which does not have a special coolant inflow structure. Nevertheless, the coolant does not flow well into the middle portion of the separator accumulating a lot of heat. In addition, the buffering region increases the size of the separator.
U.S. Pat. No. 6,924,052 discloses a separator having a cooling passage formed by etching. A large amount of coolant can be introduced into the middle portion of the separator accumulating relatively more reaction heat.
However, a high density of metal increases a weight of a stack. Meanwhile, a cooling fin can be applied to a separator. But, it increases a volume of a stack. As a plurality of separators overlap with each other, the production cost also increases. On the other hand, a thin plate can be installed between separators. However, it increases the cost and causes the structural difficulty in stack lamination.
The fuel cell includes a few components such as a membrane-electrode assembly (MEA) in which an electrochemical reaction occurs, a gas diffusion layer (GDL) which is a porous medium evenly dispersing a reaction gas onto the face of the MEA, and a separator for supporting the MEA and the GDL, delivering the reaction gas and the coolant, and collecting and transferring the generated electricity. A few tens to hundreds of components are laminated to form a fuel cell stack. An electricity generation capacity of the fuel cell increases in proportion to a reaction area of the MEA and a lamination amount of the stack. During the electricity generation of the fuel cell, hydrogen, oxygen and coolant are continuously supplied to each face of the MEA, GDL and separator. Keeping airtightness to prevent mixture of each reaction gas and the coolant is one of the most important factors in the operation of the fuel cell system.
Most of the polymer electrolyte fuel cells form an airtight structure by installing a gasket at both faces of a separator. In the case that the gasket is installed to attain airtightness, a predetermined fastening pressure is applied to the fuel cell stack to improve airtightness and electric conductivity. When such a load is applied, the GDL and the gasket are mostly deformed to obtain airtightness and electric conductivity. However, in the case of a metal separator manufactured by thin plate molding, a predetermined fastening pressure deforms part of the metal separator as well as the gasket. Particularly, the inflow portions of the reaction gas and the coolant are easily deformed due to the absence of a support member in the gasket portion and the fluid flowing portion.
Such deformation interrupts inflow of the reaction gas and the coolant, and thus applies many loads to peripheral devices, particularly, to a blower or a pump. As a result, efficiency of the system is reduced.
In the conventional fuel cell stack, the reaction gas such as a fuel gas and a deoxidization gas flows into both faces of the MEA. The metal separator separates the fuel gas, the deoxidization gas and the coolant so as to form the fuel cell stack by connecting unit cells. Here, the gasket seals up the structure to prevent leakage of the reaction gas and the coolant. As different from the graphite separator, the metal separator manufactured by thin plate molding inevitably has a complicated reaction gas inflow structure from a reaction gas manifold to a reaction gas channel so as to attain airtightness between the reaction gas and the coolant.
In order to solve the above problems, US Laid-Open Patent Gazette 20040219410 suggests a technique of coating a deformation prevention gasket on a deformation occurring portion. However, the gasket cannot completely support compression by a load. If the gasket is separated to block a reaction gas inflow portion, resistance may increase in inflow of a reaction gas.
US Laid-Open Patent Gazette 2001266911 discloses a technique of minimizing deformation of a space by adhering a metal plate. As it is difficult to mount one metal separator on the other, a manufacturing process of the metal separator is complicated. Furthermore, the added metal plate increases a weight of a stack, thereby applying many loads to a fuel cell system mounted apparatus.